Wednesday, 28 May 2014

Not much has been written about placentation in squirrels and other sciuromorph rodents - at least in comparison to myomorphs (e.g. mouse, hamster) and hystricomorphs (e.g. guinea pig, capybara). Marmots belong with chipmunks and ground squirrels in the Tribe Marmotini. The most complete description is for the thirteen-lined ground squirrel (Ictidomys tridecemlineatus) (Mossman & Weisfeldt Am J Anat 1939;64:59-109). It was based largely on specimens collected a century ago by Thomas George Lee (background here).

Interhaemal region in the labyrinth of a woodchuck placentaMarmota monax Courtesy of Dr. Allen C. Enders

The woodchuck has a labyrinthine, haemochorial placenta. In the above section the large channels with maternal red cells are lined by syncytiotrophoblast. A fetal capillary is seen to the right.

Early Development of the placenta in the Colorado chipmunkTamias quadrivittatus Courtesy of Dr. Allen C. Enders

There are several ways to make a haemochorial placenta (discussed here). Squirrels go through a transient endotheliochorial phase. The figure is from a chipmunk and shows a maternal capillary in which part of the endothelium has been replaced by syncytiotrophoblast whilst two endothelial cells remain intact (further figures here). In the first instance this creates the equivalent of the spongy zone found in other rodent placentas. Later the fetal mesoderm grows into the trophoblast bringing with it the fetal capillaries. To start with the outgrowths are fingerlike (villi) as can be seen in a recent publication on the woodchuck (here). Nearer term, however, the labyrinth occupies most of the depth of the placenta. The spongy zone is then very thin and occupied by syncytiotrophoblast with clumps of nuclei as well as mononucleate giant cells (Dr. Allen C. Enders, personal communication).Syncytins are endogenous retrovirus envelope genes (previous post). Two occur in murid rodents and another in South American hystricomorphs. Now a search of the genome of the thirteen-lined ground squirrel has turned up several candidates and further work in the woodchuck has shown one of them to be a bona fide syncytin. By in situ hybridization the gene was not expressed in the labyrinth but rather in the part of the spongy zone that had yet to be reached by the fetal vessels.

Wednesday, 7 May 2014

Derek Wildman's Group at Wayne State University have re-examined the evolution of Neotropical primates (here). Using genomic data from 36 species they derive a tree with few surprises but better support than in other recent analyses.What I found interesting is their attempt to place platyrrhine evolution in a biogeographical context. They suggest that the most recent common ancestor of extant species lived in what today is the Amazon rain forest, Guiana Shield and Northern Andes. Then, however, this was largely an area of lowlands and mountains. In support of their interpretation, known fossil sites are within this region. They further suggest that diversification of platyrrhines occurred with the establishment and development of the Amazon rain forest.
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The founder of the Neotropical primates arrived in South America during the Oligocene when the continent was completely isolated from other land masses. Personally I favour the view (espoused here) that they came by a transatlantic route. However, Jameson Kiesling et al. suggest they may have come directly from Asia. In support they cite the occurrence of all the early fossils on the west side of the continent and recent evidence that platyrrhines may have emerged in Asia rather than Africa. In either case the means of dispersal was most likely by rafting (see previous post on sweepstakes distribution).